Since lithium ion secondary batteries, which are devices having a nonaqueous electrolyte solution, can achieve high energy densities, they attract attention as batteries for cell phones, batteries for notebook computers, batteries for large power storage, and batteries for automobiles.
Although lithium ion secondary batteries can achieve high energy densities, up-sizing makes the energy gigantic, and higher safety is demanded. For example, in large power sources for power storage and power sources for automobiles, especially high safety is demanded. Therefore, safety measures are applied such as the structural design of cells, packages and the like, protection circuits, electrode materials, additives having an overcharge protection function, the reinforcement of shutdown function of separators, and the like.
Lithium ion secondary batteries use aprotic solvents such as cyclic carbonates and linear carbonates as an electrolyte solvent; and these carbonates are likely to have a low flash point and be combustible though having a high dielectric constant and a high ionic conductivity of lithium ions.
A technology is known which uses as an additive a substance reductively decomposed at a higher potential than carbonates used as electrolyte solvents and forming an SEI (Solid Electrolyte Interface) being a protection membrane having a high lithium ion permeability. The SEI has large effects on the charge/discharge efficiency, the cycle characteristics and the safety. The SEI can further reduce the irreversible capacity of carbon materials and oxide materials.
One of means to further enhance the safety of lithium ion secondary batteries includes making electrolyte solutions flame retardancy. Patent Literature 1 discloses an organic electrolyte solution secondary battery using a phosphate triester as a main solvent of an organic electrolyte solution and having a negative electrode of a carbon material as a constituting element.
Patent Literature 2 discloses that the use of a mixed solvent of a specific halogen-substituted phosphate ester compound and a specific ester compound as an electrolyte solvent can provide an electrolyte solution having a low viscosity and excellent low-temperature characteristics. Patent Literature 3 discloses a method for manufacturing a nonaqueous electrolyte battery by using a nonaqueous electrolyte solution containing vinylene carbonate and 1,3-propane sultone added therein. Patent Literature 4 discloses a battery having a nonaqueous electrolyte solution which contains a predetermined amount of phosphate esters having fluorine atoms in molecular chains thereof, and salts in a concentration of 1 mol/L or higher, has a viscosity of lower than 6.4 mPa·s. The disclosure contends that making such a constitution can provide a battery having an excellent flame retardancy, a self-extinguishing property and high-rate charge/discharge characteristics.
Patent Literature 5 discloses a nonaqueous electrolyte solution containing at least one phosphate ester derivative represented by a predetermined formula, a nonaqueous solvent and a solute. Patent Literature 6 discloses that the use of a fluorophosphate ester compound as a nonaqueous electrolyte solution can provide an electrolyte solution being excellent in the conductivity and the reduction resistance, and developing a high flame retardancy even in a low blend amount.
Patent Literature 7 discloses a nonaqueous electrolyte solution obtained by dissolving a lithium salt in a nonaqueous solvent containing a phosphate ester compound, a cyclic carbonate ester containing a halogen, and a linear carbonate ester.